1. Field of the Invention
This invention relates to the dynamic random access memories (DRAM) and more particularly to the structures and methods of fabrication of DRAM's storing multiple data bits per DRAM cell.
2. Description of Related Art
The fabrication and structure of DRAM cells and DRAM arrays are well known in the art. Typical cell structures for high density DRAM in prior art is composed of one transistor M.sub.1 70 for switching charges and one storage capacitor C 65 for storing charges is illustrated in FIGS. 1a and 1c. The transistor M.sub.1 70 will be an n-MOS transistor fabricated as shown in FIGS. 1b and 1c. A deep N-well 10 will be formed in a p-type substrate 5. The area for DRAM cells will be formed as openings during the formation of insulation. The insulation is formed by the local oxidation of the silicon substrate (LOCOS) 20. Within the deep n-well 10 a shallower p-well 15 will be formed. The gate 40 of the n-MOS transistor M.sub.1 70 will be formed as a conductive material such as polysilicon placed over an insulating gate oxide 35 to define the channel area that will be between the drain 30 and source 25 of the n-MOS transistor M.sub.1 70.
The drain 30 and the source 25 will be formed by masking the semiconductor substrate 5 and implanting an N.sup.+ material to form the N.sup.+ drain 30 and the N.sup.+ source 25. The gate 40 of the MOS transistor M.sub.1 70 will be connected to the word line control circuitry (not shown).
The capacitor C 25 is formed by placing a conductive metal connected to the substrate biasing voltage source V.sub.ss on a dielectric placed over the N.sup.+ drain of the transistor M.sub.1 70. The capacitor C 25 as shown is diagrammatic. The particular structure of the capacitor C 25 is well known and shown in "The Evolution Of DRAM Cell Technology" by B. El-Kareh et al., Solid State Technology, May 1997, pp. 89-101. In order to maintain the minimum storage capacitance of 30-40 fF of a cell, the structure of the DRAM cell results in complex semiconductor processing to develop these structures.
Refer now to FIGS. 1c and 1d, The trench capacitor C.sub.T 165 is formed by etching a deep trench 167 in the surface of the semiconductor substrate 105. An N.sup.+ material is implanted in the surface of the trench 167 and to the N.sup.+ drain 130 to form an N.sup.+ strap 152. An insulating material such as oxidized silicon nitride ONO, silicon dioxide, or silicon nitride will then be deposited on the surface of the trench 167 to form the capacitor dielectric 155. A conductive material such as polysilicon will be deposited in the trench 167 to fill the trench 167. The polysilicon "plug" 160 will then be attached to the substrate biasing voltage source V.sub.ss 175 to form a bottom plate of the capacitor C.sub.T 165. The top plate of the capacitor C.sub.T 165 will be the N.sup.+ diffusion 150 that is connected by the N.sup.+ strap 152 to the N.sup.+ drain 130 of the MOS transistor M.sub.1 170. Again FIG. 1d is diagrammatic. The particular structure is well known in the art and illustrated in B. El-Kareh et al.
The deep n-well 110 is typically biased to the power supply voltage source V.sub.cc (i.e. the highest potential on chip) and the p-well is biased to substrate biasing voltage source V.sub.ss 175 (i.e. the lowest voltage on chip). The substrate biasing voltage source V.sub.ss 175 may be biased below ground (i.e. negative potential) so that the leakage current through the pass transistor is reduced. The presence of charge in the storage capacitor C 165 indicates a logical "1" and its absence of charge indicates a logical "0". The storage capacitor C 165 is connected to N.sup.+ drain 130 of the transistor M1170, and the other N.sup.+ source 125 of the transistor M1170 is connected to bit-line V.sub.bit 180 that controls the reading and writing of the DRAM cell. The gate 140 of the MOS transistor M.sub.1 170 is connected to the word line V.sub.word 185 to control the selection of the DRAM cell.
U.S. Pat. No. 4,896,197 (Mashiko) describes a DRAM cell incorporating a trench capacitor to store one bit and a stack capacitor to store a second bit. A first pass transistor controls the charge of the trench capacitor, while a second pass transistor controls the charge of the stacked capacitor. The trench capacitor and the stack capacitor have a common L shaped plate between them. The L shaped common plate would be coupled to the substrate biasing voltage source V.sub.ss of FIGS. 1a and 1c.
U.S. Pat. No. 5,066,608 (Kim et al.) describes a method for manufacture of a stacked-trench capacitor. The stacked-trench capacitor will incorporate both a trench capacitor and stack capacitor to increase the capacitance of the DRAM cell.
U.S. Pat. No. 5,217,918 (Kim et al.) discloses an integrated semiconductor memory device incorporating stacked capacitors and combined stack-trench capacitors to form column and rows of memory cells. The stacked capacitors and the stack-trench capacitors are alternated to allow improved density, while preventing leakage current and soft errors. Again the stack capacitor and the stack-trench capacitor have a common plate that would be connected to the substrate biasing voltage source V.sub.ss of FIGS. 1a and 1c.
U.S. Pat. No. 5,234,854 (An et al.) describes a method for manufacturing a stack-trench capacitor.
U.S. Pat. No. 5,410,509 (Morita) discloses a DRAM array employing a combination of stacked capacitor memory cells and trench capacitor memory cells to form the array. The stacked capacitor memory cells will occupy on column of the array while the trench capacitor memory cells will occupy alternate columns of the memory array. The memory array will also have the stacked capacitor cells and the trench capacitor memory cells used as dummy memory cells within the array. The structure of the memory array is such that when the stacked capacitor memory cells are selected, the dummy stacked memory cells are selected and like wises for the trench capacitor memory cells and the dummy trench capacitor memory cells.
U.S. Pat. No. 5,455,192 (Jeon) describes a method of fabricating a DRAM cell incorporating stacked capacitors and trench capacitors. The structure of the stacked capacitors and the trench capacitors is such that the stacked capacitors and the trench capacitors are connected in parallel to increase the capacitance of the DRAM cell.